High count particle conversion
The usual way to improve efficiency in such case, would be to use planes with leaves texture as leaves particles (this way you already have 40 times less polys for your comp to process). You could also restrict particle emission to the part of hedge, that is actually in the camera view to avoid too big number of particles. Creating branches with leaves on them and then using this branches as particles would be some solution too. By converting praticles to objects and joining them into one mesh you will also achieve better efficiency.